In the rapidly-expanding market for office communications, computer interconnections, local area networks, and factory automation--to name a few examples--fiber-optic point-to-point data links offer a cost-effective and functionally superior alternative to copper interconnections. Multimode fiber-optic data links are intended for applications in which distances are relatively short (up to a few kilometers in length), and where cost, size, and reliability are paramount. Such requirements are characteristic of local area networks or campus computing facilities, where moderate amounts of information are transmitted among several locations. This is in contrast to the long-haul transmission market, which involves extremely high capacity and long distances. As a result of the different requirements, short-haul data links generally use low-cost light-emitting diode (LED) sources, multimode fiber, and simple junction photodetectors. In contrast, long-haul applications use semiconductor lasers, single-mode fiber, and more complex receivers.
Single-mode fiber transmission systems often use one fiber and some form of optical or electronic bidirectionality to achieve bidirectional communications. In contrast, bidirectional communications via short-haul data links are presently effected through the use of a pair of multimode fibers, one for each direction of transmission. Most commonly, identical LED transmitters are used with both fibers, so that both fibers transport light of the same wavelength. The wavelengths that are commonly used are either the 0.87 .mu.m wavelength radiation emitted by gallium aluminum arsenide (GaAlAs) diodes or the 1.31 .mu.m wavelength radiation emitted by indium gallium arsenide phosphide (InGaAsP) diodes.
The components and the operation of single-mode fiber transmission systems are expensive; in contrast, those of the multi-mode data links are relatively inexpensive. However, some of this cost benefit of short-haul data links is offset by the need for two data links to construct a bidirectional communications system. It would be advantageous if this benefit offset could be avoided.